Laser fusion and extreme field physics

Abstract:

The optimisation of laser pulse shapes and target configurations is central to high-performance inertial confinement fusion (ICF) implosions, yet remains challenging due to the high dimensionality of the design space and the substantial computational and experimental cost of evaluation. This work presents, to our knowledge, the first comparison of GP-based Bayesian optimisation and particle swarm optimisation (PSO) frameworks augmented with physics-motivated extensions in full radiation-hydrodynamic ICF optimisation under experimentally relevant constraint handling. These methods are first applied to the reoptimisation of low-convergence-ratio (Low-CR) wetted-foam implosions, providing a benchmark against traditional sequential scan approaches. Both strategies identify improved designs, with Bayesian optimisation achieving the highest final performance using fewer simulations, while PSO converges more rapidly in wall-clock time. The PSO framework is then extended to a 16-dimensional fast ignition design problem, where Gaussian process-based Bayesian optimisation becomes computationally impractical. In this regime, PSO efficiently identifies a compressed fuel assembly with ρR ≈ 1.5 g/cm2 under strict laser intensity and energy constraints. These results demonstrate that the presented optimisation strategies outperform conventional scan-based approaches and provide a scalable platform for high-dimensional ICF optimisation. Beyond numerical design studies, the same frameworks are directly applicable to experimental optimisation campaigns on high-power laser facilities, where limited shot availability and high evaluation cost demand efficient search methodologies.

Abstract:

Bright high harmonic radiation from relativistically oscillating laser-plasmas offers a direct route to generating extreme electromagnetic fields. Theory shows that under optimised conditions the plasma medium can support strong spatiotemporal compression of laser energy into a Coherent Harmonic Focus (CHF), delivering intensity boosts many orders of magnitude above that of the incident driving laser pulse [1–4]. Although diffraction-limited performance [5] (spatial compression) and attosecond phase-locking [6] (temporal compression) have been demonstrated in the laboratory, efficient coupling of highly relativistic laser pulse energy into the emitted harmonic cone has not been realised to date. Here, conclusive evidence confirms that the relativistic laserplasma interaction can be tailored to deliver the maximum conversion efficiencies predicted from simulations. By fine-tuning the temporal profile of the driving laser pulse on femtosecond (fs, 10−15 s) timescales, energies > 9 mJ between the 12th and 47th harmonics (18 eV to 73 eV) are observed. These results are shown to be in excellent agreement with the theoretically expected efficiency dependence on harmonic order, indicating that optimal conditions have been achieved in the generation process. This is the important final element required to achieve the expected intensity boosts from a CHF in the laboratory. Although obtaining spatiotemporal compression and optimal efficiency simultaneously remains challenging, the path to realising extreme optical field strengths approaching the critical field of quantum electrodynamics (the Schwinger limit at > 1016V/m or > 1029 W cm−2 ) is now open, permitting all-optical studies of the quantum vacuum and drawing new horizons for intense attosecond science.

Abstract:

Bright harmonic radiation from relativistically oscillating laser plasmas offers a direct route for generating extreme electromagnetic fields. Theory predicts that under optimized conditions, the plasma medium can support strong spatiotemporal compression of laser energy in a coherent harmonic focus (CHF), delivering intensity boosts many orders of magnitude greater than the incident driving laser pulse^1,2,3,4. Although diffraction-limited performance⁵ (spatial compression) and attosecond phase locking^6,7,8 (temporal compression) have been demonstrated experimentally, efficient coupling of relativistically intense laser pulse energy into the emitted harmonic cone has not been realized so far. Here we demonstrate that this highly nonlinear interaction can be tailored to deliver the maximum conversion efficiencies predicted from simulations. By fine-tuning the temporal profile of the driving laser on sub-picosecond (<10⁻¹² s) timescales, energies >9 mJ between the 12th and 47th harmonics are observed. These results are in agreement with the theoretically expected efficiency dependence on harmonic order, verifying that optimal conditions have been achieved in the generation process. This is the important final element required to achieve the expected intensity boosts from a CHF in experiments. Although obtaining spatiotemporal compression and optimal efficiency simultaneously remains challenging, the path to realizing extreme optical field strengths approaching the critical field of quantum electrodynamics (the Schwinger limit at >10¹⁶ V cm⁻¹ or >10²⁹ W cm⁻²) is now open, permitting all-optical studies of the quantum vacuum and new frontiers for intense attosecond science.

Abstract:

The global commissioning of multi-Petawatt laser systems provides unprecedented access to ultra-high electromagnetic fields for probing the quantum vacuum. However, current analytical models are limited, necessitating large-scale simulations for experimental validation. Here, we present real-time three-dimensional simulations of two quantum vacuum effects, using a semi-classical numerical solver based on the Heisenberg-Euler Lagrangian. The simulation model is benchmarked against vacuum birefringence analytical results with a counter-propagating setup. Simulations results of both plane-wave and Gaussian pulses are consistent with theoretical predictions. The solver is then applied to four-wave mixing using three Gaussian pulses with real-time information on the harmonic evolution. We provide quantitative explanations for the astigmatism in the output and produce precise estimates of the interaction time and size. Results are compared with the plane-wave model and previous numerical results. This solver paves the way for in-depth investigations of a broad spectrum of quantum vacuum effects in any arbitrary laser setup.

Abstract:

There is renewed interest in direct-drive inertial confinement fusion, following the milestone December 2022 3.15 MJ ignition result on the National Ignition Facility. A key obstacle is the control of the two-plasmon decay instability. Here, recent advances in inhomogeneous turbulence theory are applied to the broadband parametric instability problem for the first time. A novel dispersion relation is derived for the two-plasmon decay in a uniform plasma valid under broad-bandwidth laser fields with arbitrary power spectra. The effects of temporal incoherence on the instability are then studied. In the limit of large bandwidth, the well-known scaling relations for the growth rate are recovered, but it is shown that the result is more sensitive to the spectral shape of the laser pulse rather than to its coherence time. The range of wavenumbers of the excited plasma waves is shown to be substantially broadened, suggesting that the absolute instability is favoured in regions further away from the quarter critical density. The intermediate bandwidth regime is explored numerically – the growth rate is reduced to half its monochromatic value for laser intensities of 1015 W/cm2 and relatively modest bandwidths of 5 THz. The instability-quenching properties of a spectrum of discrete lines spread over some bandwidth have also been studied. The reduction in the growth rate is found to be somewhat lower compared to the continuous case but is still significant, despite the fact that, formally, the coherence time of such a laser pulse is infinite.